Workflows are playing a crucial role in e-Science systems. In many cases, e-Scientists need to do average case estimates of the performance of workflows. Quality of Service (QoS) properties are used to do the evaluation. We defined the Bell-Curve Calculus (BCC) to describe and calculate the selected QoS properties. The paper presents our motivation of using the BCC and the methodology used during the developing procedure. It also gives the analysis and discussions of the experimental results from the ongoing development.